Abnormal hemoglobins are sought in blood samples obtained from patients with unexplained polycythemia or anemia. Oxygen affinity and stability, as well as electrophoretic and chromatographic mobility, are measured in the screening process. When new or unusual findings are encountered, appropriate clinical, genetic, hematologic, and biochemical studies are performed. Oxygen transport to tissues will be studied in patients with disorders of red cell mass, particularly sickle cell anemia, and the effect of usual therapeutic procedures will be evaluated by measurements of cardiac output, oxygen consumption, and mixed venous pO2, using non-invasive procedures whenever possible. Muscle capillarity, and mitochondrial function, will be assessed in small biopsies from patients.